I suspect the packager decided to make all the PIM applications all-or-nothing. I also dislike kmail intensely and don't want it on my system, since it lost large numbers of e-mails and I spent a couple of days recovering as many as I could. I don't trust it and I don't want it, but I'm stuck with it too, in my case because I use kalarm.
I don't know what has changed here since the previous version of KDE, so I can't hazard a guess as to whether this is a packaging error or something forced on us by KDE, who seem to be becoming increasingly dictatorial and MS-like at the moment.